1. Field of the Invention
The present invention relates to the measurement of liquid volumes using spectrophotometers. More particularly, the present invention relates to systems and related methods to test or calibrate liquid delivery devices using a combination of vertical spectrophotometry and horizontal spectrophotometry.
2. Description of the Prior Art
All currently manufactured colorimetric systems for testing or calibrating liquid delivery devices (e.g., the PCS® system and the MVS® system, both offered by Artel, Inc. of Westbrook, Me., assignee of the present invention and application, and the Pipette Volume Calibration Kit offered by VistaLab of Mt. Kisco, N.Y.) use a multiplicity of sample solutions for the purpose of testing a wide range of deliverable solution volumes. The sample solution is delivered by the device being tested into a diluent in a measurement vessel, and the solutions are mixed before measuring the absorbance of the resulting mixture. A concentrated sample solution is used to test a small liquid delivery volume, and a more dilute sample solution is used to test a large liquid delivery volume. As used herein, a “vessel” is any vial, cell, bottle, microtiter plate or other type of container for retaining a fluid therein, whether such vessel is sealed or not. Also, the vessel may be designed for collecting optical absorbance measurements by use of a horizontal beam spectrophotometer (such as a conventional UV-Vis spectrophotometer like the Cary 5000, Varian, Inc., Palo Alto, Calif.) or a vertical beam spectrophotometer (such as a microtiter plate reader like the ELx800, BioTek Instruments, Winooski, Vt.).
The Artel PCS is a photometric method and system for calibration of pipettes or other liquid delivery systems. Its advantages are highly accurate and reproducible measurement results which are traceable to national standards, simplicity of use, independence from environmental factors such as vibrations, temperature variations, and drafts, and applicability to very low (microliter) liquid dispense volumes. Its disadvantages are relatively high cost of consumables, since the diluent solution is packaged in a clear disposable vial which needs to be manufactured to high optical standards, and the need for operator intervention to change to a fresh vial of diluent after a certain number of sample additions (between 11 and 40 dispenses, depending on the sample size and concentration). One important element in achieving the high degree of accuracy and precision of the Artel PCS is the use of a horizontal beam spectrophotometer which measures absorbance at two wavelengths and which has extremely low noise and high accuracy (noise typically better than 2×10−5 absorbance units at an absorbance of 1).
The Artel MVS is a photometric measurement method and system for calibration of multichannel pipettes and automated liquid delivery devices. It is based on a specialized microtiter plate to which both diluent and sample solutions are added. It employs a vertical beam spectrophotometric system to measure absorbance at two wavelengths. The volume of sample solution can be calculated knowing the dimensions of the wells in the microtiter plate and measured absorbances. Its advantages are: adaptability to measuring delivery volume from multichannel dispensers (up to 384 dispenses at once), speed, results which are traceable to national standards, and independence from operator technique or environmental variables. Its weaknesses are high consumable costs (primarily for the specialized microtiter plate), and a limitation that only one sample volume can be delivered per well of the microtiter plate.
Many applications, such as the automated testing of liquid delivery devices, are preferably done automatically, without operator intervention. It should be possible to schedule and carry out testing operations throughout the day on an automated basis with no operator attention or intervention. Neither of the above described existing-art methods fulfill this need, as both require operator intervention to change out the disposable vial or microtiter plate. For many critical applications in which it is important to verify liquid handler performance on a frequent basis, cost of consumables for either of the above methods becomes prohibitive. What is needed is a liquid delivery calibration system that does not require operator intervention and that has significantly lower consumables cost per test. What is also needed is a liquid delivery calibration system that minimizes the need for operator time and equipment time in the procedure.